The present invention relates generally to apparatuses for reproducing a video signal of a certain system recorded on a magnetic tape as a video signal of a different system. More specifically, the invention relates to an apparatus for reproducing a video signal recorded on a magnetic tape as a video signal of one out of a plurality of different systems which are classified according to difference in the field frequency and the number of horizontal scanning lines as a video signal substantially of another system.
At present, there are two principal video signal systems for television. One system, used in the United States, Japan, and other countries (hereinafter referred to as the "first standard system" or the "first system") is based on a field frequency of 60 Hz (precisely speaking, 59.94 Hz in the case of a color video signal) with a number of horizontal scanning lines of 262.5 lines within one field. The other system, used in Europe and other areas (hereinafter referred to as the "second standard system" or the "second system") is based on a field frequency of 50 Hz with 312.5 horizontal scanning lines within one field. There are various other systems such as those of combinations of field frequencies and numbers of horizontal scanning lines within one field of 50 Hz and 202.5 lines and 50 Hz and 409.5 lines. However, with the full-scale spread of color telecasting, the trend of intensification is toward the above mentioned first and second standard systems.
As is known, video signal systems can be further classified by the mode of transmission of the chrominance signal as the NTSC system, the PAL system, and the SECAM system in addition to the above described classification based on differences in field frequency and number of horizontal scanning lines within one field. With respect to this difference in mode of transmission of the chrominance signal, mutual conversion can be carried out with relative ease by an electrical process.
However, so-called system conversion, wherein field frequency and number of horizontal scanning lines within one field are converted, has heretofore required very complicated and elaborate apparatus. More specifically, the necessity for so-called system conversion, wherein field frequency and number of horizontal scanning lines are converted, has heretofore been primarily a problem mutually between telecasting stations or networks as in international relay telecasting between different regions (for example, between Europe and the United States). For this reason, it has been necessary to carry out this system conversion in a manner to meet very strict standards. Consequently, elaborate and large-scale electronic conversion apparatuses have been employed.
In an advanced electronic conversion apparatus being used at present, a system by which a video signal is once converted into a digital signal, which is then stored in a digital memory of large capacity, subsequently read out in a required sequence, and restored into the original video signal is used. By manipulating the time ratio of this signal storing and reading out, conversion of field frequency and number of horizontal scanning lines is accomplished. This electronic conversion apparatus, however, is diadvantageously elaborate and expensive.
Prior to the completion of electronic conversion apparatuses of this character, a number of other apparatuses has been devised and reduced to practice. In a most simple apparatus, the picture due to one video signal system is formed on the screen of a cathode-ray tube and optically picked up by the television camera of the system of another second video signal, which second video signal is thereby obtained.
In another system which has been devised, a video signal magnetically tape recording and reproducing apparatus (hereinafter referred to as VTR) is utilized to carry out repeated recording and reproducing, and by staggering the time-axis relationship between recording and reproducing, system conversion is accomplished. This system, however, has been accompanied by the following problems. In this system, conversions of field frequency and of the number of horizontal scanning lines are carried out separately, and a system conversion is carried out by providing three kinds of VTRs according to purpose, and recording and reproducing are repeated three times. Consequently, this system requires an elaborate and large-scale apparatus. Moreover, since magnetic recording and reproducing are newly repeated three times especially for system conversion, the signal to noise ratio and frequency characteristic of the video signal deteriorate remarkably.